During my research for the "field uniforms" of the British Army 1815 I came across a note the acknowledged researcher Rev. Sumner left in his notes / sketches, now in possession of the ASKB.
He wrote in the section covering the Foot Guards: "Many men of the Foot Guards at Waterloo fought in their Surrey Militia jackets (Cotton, "Voice from Waterloo", page 10).
The mentioned text of Edward Cotton in his memoirs is written on page 7 of 1854 edition (can be downloaded via google) - he states that "a large proportion of the British troops was composed of weak second and third battalions, made up of militia and recruits, who had never been under fire*..." - the footnote says "* The 3d guards and 42nd Highlanders had near eight hundred militia-men in their ranks. The guards actually fought in their Surrey militia jackets."
Of course I checked several other sources - e.g. the three-part study of Bryan Fosten published in the series "Soldiers of the Napoleonic Wars" of Almark in the 70/80s.
Bob Burnham and Ron McGuigan give in their exceptional study about the British Foot Guards at Waterloo some strength numbers for the four battalions present in the Netherlands - with the addition of reinforcements; for the period of March to June 1815, the following numbers can be derived as "new" soldiers to the guards battalions:
2nd bat. / 1st Guards: 79 men
3rd. bat. / 1st Guards: 35 men
2nd bat. / Coldstream Guards: 264 men (!)
2nd bat. / 3rd Guards: 274 men (!)
So particularly the Coldstream and the 3rd guards got lots of reinforcements, most of them in April/May 1815.
Now the very interesting question: do you have any more hints except the quote from Edward Cotton that the foot guards (particularly 2nd and 3rd Guards) had a significant number of soldiers fighting at Waterloo in their (Surrey?) militia jackets?
This would give figure dioramas surely a nice additional note 😀
Greetings from Berlin
Markus Stein
Thank you all for your comments - indeed the coats of the Surrey Militia is quite similar to the Guards uniform due to the royal blue facings - and the prescribed pairs of buttons/laces would make it difficult for witnesses to differ them from at least the Coldstream Guards - but for the 3rd (Scots) Guards it seems to be more difficult.
I appreciate David's opinion about the regiment's tailors to change the laces and buttons - but did they have the time AND the material to do this in time for the reinforcements reaching the Netherlands especially in May 1815. I also think that only one source is quite "dangerous" to take it as proven, but perhaps Cotton got in contact to soldiers of the Guards and they told him about this feature - at least we don't know and I fear the only answer may be by consulting the archives of the Guard's regiments.
@Andrew: good point and yes, if they came from their barracks onto the Netherlands they would have had their guard's coats - unfortunately the numbers given by Bob and Ron in their book don't differentiate between the provenance of the troops.
Greetings from Berlin
Markus Stein